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Tuesday, 11 March 2014

3/3 - this, ladies and gentlemen, is the end. At least, until the inevitable prequels where I'll ignore the canon of the expanded universe and do my best to tarnish everything you liked about the original trilogy.

Here we are then, the last instalment of the Wealden Crocodyliformes Trilogy. Following the posts on atoposaurids and goniopholidids, today we're going out with a bang by covering a newly described Wealden crocodyliform unleashed on the world this morning. The study was written up by my University of Portsmouth chums and colleagues Steve Sweetman, Ulysse Pedreira-Segade and Steven Vidovic (Sweetman et al. 2014), and Steve V. has covered some aspects of his involvement at his blog. The paper is open-access so, for the full skinny on the discovery, you should head here.

This most recently identified Wealden crocodyliform is among the most sophisticated and unusual of all Wealden crocs. Named Koumpiodontosuchus aprosdokitii, it is known from a well-preserved skull which was recovered in circumstances owing much to chance and good fortune (Sweetman et al. 2014). This animal is currently only known for certain from the Wessex Formation of the Isle of Wight, specifically from fossil-rich cliffs next to the seaside village of Yaverland, and the only known skull of it is broken in half. The posterior half was discovered in March 2011 by holidaying fossil hunters, who took it to the local dinosaur museum (Dinosaur Isle, of Sandown) to have it identified. Another family, on a fossil-hunting holiday three months later, then found the front half of the skull. They took this to the same museum where, by chance, the same museum staff who’d handled the first piece were on hand. It was realised that each piece belonged to the same specimen, and the first half was rapidly brought back to the museum to check the degree of articulation. Remarkably, the join between the broken pieces was near perfect – clearly neither chunk had been exposed to weathering effects very long before being discovered – and the entire skull could be seen. Each piece was then donated to the museum to allow its study. Given the chain of events and people involved in the discovery of Koumpiodontosuchus, it’s easy to imagine how only single halves of the skull might be known to science, or even neither. This is clearly yet another story which stresses the importance of amateur fossil hunters to Wealden fossil discoveries, and the benefits of responsible collecting.

Button-toothed crocodiles in context

Koumpiodontosuchus is a member of Bernissartidae, a group named by Sweetman et al. (2014) which only contains two species: Koumpiodontosuchus and Bernissartia fagesii. The latter is a famous, small Jurassic and Cretaceous crocodyliform known from France, Denmark, Spain, Portugal and particularly Belgium, where a spectacular complete skeleton has been unearthed. Indeterminate species of Bernissartia also seem to occur in the Ashdown Formation of Hastings (Salisbury and Naish 2011), but this identification may eventually warrant reappraisal now that Koumpiodontosuchus has been discovered. Bernissartid remains are not new, some of the first material of these animals being documented in the 1850s and Bernissartia itself being named from Belgian fossils in the 1880s. Isolated teeth, likely referable to Koumpiodontosuchus, have been found in Wealden deposits since at least the 1970s (Buffetaut and Ford 1979), so were clearly present across the entire geographic and stratigraphic range of the Wealden Supergroup.

Bernissartia has long been a bit of an oddball among Crocodyliformes, possessing some unusual anatomy and being of uncertain placement in crocodyliform systematics. The discovery of Koumpiodontosuchus provided a bit of light on this front, suggesting that Bernissartia was part of a group containing at least one other similar species, and that they occupy an evolutionary place between atoposaurids and the goniopholidid + Eusuchia radiation. This position isn’t too surprising, as there are a number of features in bernissartids which link them to Eusuchia – see below. Bernissartidae is primarily defined by dental characteristics, with the most obvious one also being the namesake of Koumpiodontosuchus: “button-toothed crocodile” (if anyone wants a common name for these Crocodyliformes, this is the one to use). The posterior teeth of bernissartids are rather globose – wide, short and blunt – and distinctive compared to the dentitions of most other Crocodyliformes. It’s these teeth which, even in isolation, betrayed the presence of bernissartids in the Wealden well before the more substantial Koumpiodontosuchus fossil was discovered. Their other teeth are quite different to this, however. The mid-region dentition is rather conical in shape; ‘pseudocanines’ erupt about 25 % of the jaw length from the jaw tip, and conical teeth emerge procumbently from the jaw tips themselves. Koumpiodontosuchus has two large pseudocanines on its lower jaw, which erupt so close to each other that they share a single, enlarged tooth socket. Bernissartia, by contrast, only possesses one.

The new Wealden bernissartid Koumpiodontosuchus aprosdokitii foraging for molluscs. It's eating a mud snail, Viviparus cariniferus, while tiny (6 mm long) physid gastropods Prophysa crawl over pond scum in the lower left of the image. Dragonflies provide scale, while unnamed tetanurans (based on findings of Benson et al. 2009) prowl around the background. An earlier version of this reconstruction was featured in Sweetman et al. (2014). Prints of this image are available here.

Bernissartids packed this sophisticated dentition into relatively tiny jaws: these were not big crocodyliforms. Indeed, with body lengths of approximately 600 mm, bernissartids were probably the smallest crocodyliform species in the entire Wealden succession. Like goniopholidids, bernissartids bore osteoderm shields on their backs and bellies, but the dorsal series was rather more complex than those of other Wealden crocodyliforms. Rather than possessing two rows of interlocking osteoderms as we saw in goniopholidids and atoposaurids, bernissartids possess four rows of osteoderms along their backs. These comprise two sets of rectangular, double-keeled scutes along the midline, and laterally bordering square osteoderms with single keels (Salisbury and Frey 2001). None of these interlocked, and – based on what we’ve discussed for other Wealden Crocodyliformes – it’s worth considering what impact this had on bernissartid locomotion. Rather than supporting their trunks with scutes, it seems that bernissartids developed procoelus trunk vertebrae (that is, vertebrae with centra extending into the corpus of the vertebra behind) to support their bodies when walking (Salisbury and Frey 2001). This feature, along with their relatively complex osteoderms, is shared with eusuchians and are some of the reasons why these animals have classically been allied to these Crocodyliformes. Of further interest here is the biconvex nature of the first bernissartid tail vertebra – this has further implications for their locomotion, which we’ll get to below.

The bit on palaeoecology

Ecologically, it seems that bernissartids had a preference for hard shelled prey. Their blunt posterior dentition has been labelled as ‘tribodont’ – literally meaning ‘crushing teeth’ – and, like slamming a couple of anvils together, are ideally shaped to crunch hard shells. Some confirmation of this idea is seen in the wear facets often seen on tribodont bernissartid teeth. Classically, their prey was largely considered to comprise molluscs such as the freshwater snails and clams populating Wealden streams and lakes (Buffetaut and Ford 1979). Recently, a broader diet has been postulated for bernissartids however, the logic being that hard shells are hardly restricted to molluscs even in freshwater settings (Sweetman et al. 2014). Insects and crayfish probably formed as much of their diet as molluscs, all of which were likely procured or extracted from soft-substrates with the procumbent anterior teeth. We should not forget the savage-looking pseudocanines of these animals however: these would be of little use against hard prey items, but may have allowed for spearing relatively soft-animals. Perhaps bernissartids are best viewed as rather opportunistic feeders, primarily taking hard-shelled prey but not turning their noses to other types of food when the opportunity arose.

If gastropods like this Wealden mud snail, Viviparus cariniferus, had nightmares, they contained bernissartids.

Where was most of this prey caught? There is evidence that bernissartids were equally at home in water and on land. Their biconvex first tail vertebra suggests their tails were capable of considerable movement for providing burst propulsion through water and, unlike most other Wealden Crocodyliformes, their lack of interlocking osteoderms facilitated lateral trunk motion (Salisbury and Frey 2001). While compromising overall speed, this may have permitted greater amounts of manoeuvrability – ideal for pursing nimble, if relatively slow, aquatic arthropods. We’ve already mentioned that the reinforced trunk vertebrae of bernissartids would provide ample reinforcement for terrestrial locomotion, and their small size is relevant here as well. Like the small-bodied atoposaurids, and unlike the big goniopholidids, bernissartids had relatively small amounts of weight to lug around on land and could likely sustain long periods of terrestrial locomotion without tiring. It’s possible, therefore, that they found much of their prey on land as well as in water, perhaps enjoying the beetles, cockroaches and other tough-shelled terrestrial insects known to occur in Wealden deposits.

It’s worth pointing out that bernissartids may not be the only Wealden Crocodyliformes adapted for hard-shelled prey. The poorly known, 1.5 m long Wealden eusuchian Hylaeochampsa vectiana also has large posterior teeth ideal for smashing shelled prey (Clark and Norell 1992), although the dentitions of other hylaeochampsids are complex and it’s possible Hylaeochampsa had a very varied diet. As discussed for other Wealden Crocodyliformes, it’s likely that the size difference between the bernissartids and Hylaeochampsa would prevent too much overlap in prey preference: the latter may have been capable of eating large molluscs or even small armoured vertebrates, which were probably unavailable to bernissartids. There's lots more we could say here, but I'd best not - maybe Hylaeochampsa will warrant dedicated discussion at a later date.

The end

And I guess that's where we'll leave the Wealden Crocodyliformes for now. As alluded to above, there are other crocodyliform species and groups we could discuss, but they're generally less well known than the taxa we've covered across these posts and it would be difficult to discuss them in comparative depth. I hope you've enjoyed this series of themed posts and, if artwork of ancient Wealden animals is your thing, come back soon for a big announcement about an event related to just that.

Monday, 10 March 2014

Welcome to Episode 2 of the snappily-titled Wealden Crocodyliformes Trilogy!* We'll waste no time with introduction - read this if you haven't already - and dive straight into our second group, the goniopholidids. Much of the information herein is derived from Salisbury and Naish (2011) so, if in doubt, consult this tome for further details.

*Snappily titled? And it's about crocodiles...? Man, I'm so wasted on you guys.

Without question, the Wealden waterways were lorded over by a group of Crocodyliformes known as Goniopholididae. The namesake of this group, Goniopholis, is one of the more familiar Mesozoic crocodyliforms after famous taxa like Sarcosuchus and Deinosuchus, and is well known as a relatively ‘conventional’ crocodyliform compared to some of the other weirdo crocs doing the rounds in the Mesozoic. Goniopholidids are found throughout Jurassic and Cretaceous rocks in the Northern Hemisphere and are part of several famous fossil faunas, including being the best known crocodyliforms of the Wealden fauna. Miscellaneous goniopholidid teeth and scutes occur throughout the Wealden, and their existence has been known for a long time. Teeth ultimately attributed to indeterminate goniopholidids were found in Sussex during the 1820s by Gideon Mantell as part of the same collections of crocodile’ material which was later found to contain unappreciated early records of Wealden baryonychines.

Despite this long history, work on Wealden goniopoholids is still developing (Salisbury and Naish 2011). At one time, most Wealden goniopholid taxa were considered members of Goniopholis proper, the famous Owen-named genus of great historic significance. As with many 'classic' genera, Goniopholis is now appreciated to be a bit of a taxonomic mess and claims of 19 species are being scrutinised (e.g. Salisbury and Naish 2011; Andrade et al. 2012). Recent reviews have suggested that the Wealden goniopholidid assemblage contains a sole Goniopholis species from the Weald Sub-basin and two other genera from the Wessex Sub-basin, all known from good skull material and, in the latter instances, a series of articulated postcranial remains (Salisbury and Naish 2011). These named species include Goniopholis willetti from the Grinstead Clay Formation, Sussex; Anteophthalmosuchus hooleyi (below), from the Wessex and Vectis Formations of the Isle of Wight, and Vectisuchus leptognathus, also of the Wessex Formation (why Vectisuchus when it’s not found in the Vectis Formation? ‘Vectis’ is the Roman word for the Isle of Wight, so ‘Vectis’ frequently pops up in animal names from this part of the world). The latter was almost known from a complete skeleton, but a cliff fall during its collection rendered much of the hindlimb, pelvis and tail inaccessible. In spite of this incident, it’s still fair to say that this group has one of the better records among Wealden reptiles, and it might get even better. Fragmentary goniopholidid jaw fossils hint at further, unnamed species, but they are currently too poorly represented to warrant naming.

The Wealden goniopholidid Anteophthalmosuchus hooleyi takes advantage of a flooding river to hunt two stranded Hypsilophodon foxii. The big one is Using the Ballet to escape. Prints of this image are available here.

Goniopholidids vs. modern crocodilians, round 1: anatomy

What kind of Crocodyliformes were goniopholidids? Because these animals appear to resemble modern crocodiles in size, shape and probably lifestyle moreso than any other well-known Mesozoic Crocodyliformes, they are often reconstructed as ancient copies of large modern species like Nile or saltwater crocodiles (e.g. Karl et al. 2006 - see reconstruction here). This isn’t really the case, however: goniopholidids may look a little similar to modern crocodilians at first glance, but much of their anatomy is unconventional and their possible habits were likely rather different. If we compare these aspects directly, their differences will soon become apparent.

We’ll start with size. Here, it must be said, goniopholids are undoubtedly pretty similar to modern crocodilians. The largest Wealden goniopholidids – Anteophthalmosuchus and G. willetti - were large animals each attaining at least 3.5 m long. This is a pretty comparable size for many modern crocodiles, and may even seem a little on the small side compared to the 5 m+ lengths attained by some extant crocodilians. Don’t be fooled into thinking this makes Wealden goniopholids diminutive creatures, however: a 3.5 m long crocodyliform would somewhere around 200 kg in weight and stretch longer than your 3-seater sofa. These were undoubtedly big, bulky animals. Vectisuchus, by contrast, was a much smaller species, only attaining 1.2 m in length.

In fine anatomy, we start to see obvious differences between the ancient goniopholids and modern crocodilians. Goniopholidid backs were covered with two rows of rectangular osteoderms with interlocking pegs at their distal margins – they are much like the atoposaurids we met last time in this respect (Salisbury and Frey 2000). These are largely devoid of ornamentation with only slight keels along their dorsal surfaces. This configuration is rather different to the more complex and ornate osteoderm arrangements seen in modern crocodyliforms, and goniopholidid osteoderm shields would probably seem rather simple and inelegant by contrast. Also unlike modern crocs, goniopholid osteoderms do not extend far up the neck, perhaps because doing so would impair neck mobility (see below), and further osteoderms were found along their bellies. These were formed of hexagonal plates rather than long, rectangular ones however. Another key distinction between goniopholidids and modern crocodilians is found in their forelimbs. Most goniopholids have arms which are at least as long as their legs and many species - including Anteophthalmosuchus and Vectisuchus – have forelimbs which surpass the length of the hindlimb. This increased length is provided by relatively elongate humeri and wrist bones, and would give goniopholidids taller statures than those of all modern crocs, which are always shorter up front than behind. If we extending this comparison further, we’ll see that goniopholidid forelimb length is almost unique among all Crocdyliformes, being longer than virtually all of their relatives.

Like many modern crocodilians, goniopholidids possess the well-built, powerful skulls of formidable predators. There is also overlap in general skull shape with modern crocs too, with G. willetti and Vectisuchus having rather long, slender snouts which are narrower than the posterior regions of their jaws. By contrast, the skull of Anteophthalmosuchus belongs to a real bruiser; its jaws only gently converge from the enormous posterior region to form a chunky, roughly triangular skull with a rounded muzzle. Both skull types are equipped with goodly-sized, slightly recurved conical teeth which would not look out of place on modern crocodilians. Again however, there are differences in detailed anatomy. Of particular interest is the orbits of Anteophthalmosuchus, which only permitted forward vision rather than anterolateral as is usual for Crocodyliformes (its name, roughly meaning ‘forward-eye-crocodile’, reflects this - see Salisbury and Naish 2011). A similar condition is also seen in Vectisuchus, but it is not quite as well developed and seems to have arisen independently. Goniopholid skulls are also rather flatter than those of modern crocs, and have distinctly over-biting upper jaws. An unusual hollow in the cheek region, known as the maxillary depression, was also present, apparently representing an unusually large pressure-sensitive region of the goniopholidid face (Andrade 2009).

A house-proud Goniopholis willetti stands at the entrance to his burrow. Note his long arms, narrow jaws, and lack of a doormat.

Goniopholidids vs. modern crocodilians, round 2: habits

It may be expected that these similarities and differences between modern crocodilians and goniopholididis may translate to overlapping, but also slightly different lifestyles. Happily, because the anatomy of Wealden goniopholidids is well-documented, we can make some informed speculation as to how these animals may have lived and, indeed, this seems to be the case. The size and robust skeletons of G. willetti and Anteophthalmosuchus suggests that the ecological bucks of Wealden waterways stopped with them: occasional visits from spinosaurids aside, they were the largest predators in Wealden lakes and rivers and clearly well suited for tackling large prey items. We might imagine each as the apex predators of their respective waterways, taking small or medium-sized terrestrial animals, large fish and other aquatic reptiles as prey. This gives these animals a role much like those filled by several species of large crocodilians today. Smaller Vectisuchus, by contrast, probably ranked it in the mid-league of ancient Wealden ecosystems, probably capable of holding its own against most aquatic Wealden species but wanting to be wary of its larger cousins. Applying trends of snout shape and prey preference of modern crocodiles to Wealden goniopholidids suggests they likely differed in general prey preference: slender-snouted Vectisuchus and G. willietti probably took relatively smaller prey than the massively-jawed Anteophthalmosuchus. Through overall body size and jaw shape, these animals probably avoided stepping on each other’s ecological toes – at least Anteophthalmosuchus and Vectisuchus were contemporaries which probably practised niche partitioning (Salisbury and Naish 2011).

We might expect goniopholidids to exploit their large size in a similar way to modern crocodilians. Large modern crocodiles often focus their predation efforts to certain times of year when environmental conditions are favourable, such as times when rivers and lakes are in flood, when prey is particularly abundant, or at least the climate is more forgiving. Given how extreme the Wealden climate was - summer temperatures in some parts of the Wealden reached 36–40°C and experienced annual droughts (Sweetman and Insole 2010) – goniopholidids may have used similar strategies. As with big modern crocodilians, their large bodies hold ample reserves to wait out leaner or stressful times, and it’s possible that some goniopholidids waited out the long, hot Wealden summer in cooling pools or burrows (see image, above), while smaller crocs had fewer resources to fall back on and continued to exert themselves throughout hard times.

Beyond these generally favourable comparisons however, many aspects of goniopholidid anatomy hint at different habits to modern crocodilians. For instance, the development of goniopholidid maxillary depressions likely represent enlargements of sensory organs present in modern crocodilians used to detect prey at the water/air interface (Andrade 2009). All else being equal, does this indicate that goniopholids were more routinely grabbing prey at the water surface rather than diving for food or living generalist lifestyles? In other instances, it’s not clear what significance goniopholidid anatomical quirks may have. It’s difficult not to wonder why some Wealden goniopholidids possess entirely forward-facing eyes, for instance, and if this was related to feeding. Ordinarily, increased amounts of forward vision are associated with development of binocular vision and heightened abilities to judge distances. Might that mean predation techniques were unusual in some goniopholidids, involving chases, or carefully judged lunges and strikes at prey?

The preferred habitats and locomotory methods of goniopholidids are also worth pondering. There is some evidence that larger Wealden goniopholidids were mostly confined to a semi-aquatic existence, as their interlocking osteoderms likely strengthened their backs and improved terrestrial competency (as it does for atoposaurids and several other type of ancient crocodyliform), but their sheer weight likely impeded terrestrial locomotion over sustained periods (Salisbury and Frey 2000). The same is true of large modern crocodiles: here, reinforced vertebral joints perform a similar job to osteoderm bracing but still fail to facilitate effective, fast terrestrial locomotion for long periods. Larger crocodilians therefore spend much of their time in water, and certainly find most of their food there. If so, this makes the atypically long forelimbs of goniopholidids all the more interesting. Often, development of relatively equate limb lengths in quadrupeds is considered a sign of good terrestrial proficiency, betraying a well-balanced animal with effective carriage and equal gait efficiency in both limb sets. But how can this apply to large, heavy goniopholidids if they weren’t walking very much? Doubtless, an increased forelimb stride length was useful on occasions when large goniopholidids did leave the water, but why develop these features if much of their lives were spent in deep water? Did these animals ‘walk’ along river beds more than other Crocodyliformes? Was this trait even important for big adults? Perhaps smaller or juvenile goniopholidids took advantage of long forelimbs before they outgrew real terrestrial proficiency, spending more time on land before becoming more thoroughly aquatic at larger sizes. We could speculate all night about the intriguing possibilities here: Crocodyliformes are sophisticated creatures which do a lot more than eat, sleep and wander about: they also dig burrows, construct nests, climb onto trees and rocks, and are very sociable. Could their long forelimbs be related to these behaviours? Vertical size is seemingly more intimidating to modern Crocodyliformes than girth (Farlow and Dodson 1975) - might long arms and a tall stature have incurred social significance for goniopholidids? It’s not inconceivable that the long arms of goniopholidids were influenced by these activities rather than just locomotion, and I suspect an investigation into the evolution and functionality of their forelimbs would yield some very interesting results.

The outcome

In sum, then, it seems that we need to be cautious when thinking of goniopholidids as 'conventional' Crocodyliformes or simply forebears of modern crocodiles. Many aspects of their anatomy are not only different from those of modern crocodiles, but actually downright odd, and likely impacted on their habits and lifestyles significantly. Palaeoartists - bear all this in mind the next time you set out to draw your goniopholidids skulking in the background of your dinosaur artwork.

For the concluding post in the Wealden Crocodyliformes Trilogy, we're going out with a bang and an exciting new discovery - and it's not very far off now. Stay tuned!

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Palaeontologist and palaeoartist, based at the University of Portsmouth and specialising in flying reptiles. Check out my work at MarkWitton.com, follow me on Twitter @MarkWitton, and buy my book here. Contact me about my work at wittonprints@gmail.com.